This invention relates to a method for increasing the economic value of breeding stock semen.
In mammals, the sex is determined by two different types of sperm, which have either an X-chromosome (X-sperm) or Y-chromosome (Y-sperm). The economic impact of sex preselection in livestock is well documented. See "Impacts of Applied Genetics, Micro-Organisms, Plants and Animals" Chapter 9, 1981, OTA Report. Lib. Congress Cat. Card No. 81-600046; R. J. Gerrits et al, "Economics of Improving Reproductive Efficiency in Farm Animals".
In U.S. Pat. No. 4,009,260, I claim methods for increasing the incidence of males in the offspring of mammals by artificially inseminating a fertile female with a Y-sperm enriched sperm fraction of that mammal obtained by maintaining at least the sperm portion of progressively motile sperm-containing semen, either as such or suspended in an aqueous suspending vehicle physiologically acceptable to the sperm, as an upper layer in vertical interfacial contact, at a temperature at which the motile sperm are motile, with a lower discrete layer of a first aqueous contacting medium physiologically acceptable to the sperm and in which the motile sperm migrate downwardly at a slower rate than the upper layer, until a portion only of the motile sperm of the semen has migrated downwardly into the contacting medium, thereby producing a contacting medium containing a higher proportion of Y to X sperm than in the starting sperm, and repeating the step at least once, employing in the upper layer the motile sperm which have migrated to the first aqueous contacting medium and as the lower layer of second aqueous contacting medium physiologically acceptable to the sperm and in which the motile sperm migrate downwardly at a slower rate than in the first contacting medium.
This method has gained acceptance in human clinics but its usage in animal husbandry has been limited by the fact that a significant proportion of the motile sperm of the fractionated semen is "wasted", i.e., that portion not in the Y-sperm enriched fraction or fractions. In other words, the economic gain achieved by producing a higher incidence of male offspring is partially offset by the inherently higher cost of Y-sperm enriched portion of the breeding stock sperm resulting from the "wasting" of a portion thereof in the patented process as well as the cost of fractionating the sperm to obtain an X-sperm enriched fraction.
Such a method would be economically more attractive for use with livestock herds if the number of inseminations achievable from an ejaculate were not substantially reduced by the patented fractionation process. However, the patented method is directed to maximizing the ratio of motile sperm to immotile sperm rather than maximizing the recovery of starting motile sperm since, in human beings at least, all other factors being equal the higher the ratio of motile sperm to immotile sperm in the seminating fluid, the greater the likelihood of a successful insemination therewith. In maximizing the ratio of motile to immotile sperm, a significant portion of the starting motile sperm is inherently lost.
In U.S. Pat. No. 4,339,434, I claim a method of increasing the likelihood of conceiving a female fetus by promoting ovulation in a fertile female mammal with an ovulation-inducing agent and then artificially inseminating the female mammal during the period of expected ovulation with a sperm fraction of enhanced sperm motility from which immotile sperm and non-sperm seminal components have been separated and which is suspended in serum albumin or like physiologically acceptable vehicle. This process also inherently "wastes" a portion of the starting motile sperm, since some of the motile sperm remains in the fraction containing the immotile sperm and non-sperm seminal components.
In U.S. Pat. No. 4,007,087, I claim a method for enhancing the survival rate of sperm subjected to frozen storage by subjecting sperm to the above-described fractionation methods prior to freezing. However, that invention makes no use of the motile sperm which is not present in the motile-sperm enriched fraction.
I have found that my above-described fractionation processes can be employed to enhance the total economic value of breeding stock semen, beyond that achievable when only the motile-sperm enriched the fraction thereof is employed.
The successful use of both the fraction of sperm enriched in motile sperm and the fraction of sperm separated from the motile enriched fraction, in accordance with the instant invention, to achieve a higher incidence of female offspring, respectively, in sheep has been reported by I. G. White, et al. in "Reproduction in Sheep", Lindsay, D. R. and Pearce, D. T., Supervising editors, Brinda Fulla Press and Pub. Pty. Ltd., Canberra ACT Australia, pp. 299-300 (Dec. 1984).